1Field of the Invention
The present invention relates to a control apparatus for an engine.
2. Discussion of Background
FIG. 1 is a block diagram showing the construction of a typical fuel control apparatus for an engine wherein an air flow sensor (AFS) for detecting an intake air quantity is used. In FIG. 1, a reference numeral 1 designates an air cleaner, a numeral 2 a hot wire type AFS, a numeral 3 a throttle valve for controlling the intake air quantity to the engine, a numeral 4 a surge tank, a numeral 5 an air intake manifold, a numeral 6 an air intake valve driven by a cam (not shown), and a numeral 7 a cylinder. Although FIG. 1 shows only a single cylinder for simplifying explanation, the engine is, in fact, constituted by a plurality of cylinders.
A numeral 8 designates an injector attached to each of the cylinders and a numeral 9 an electronic control unit (hereinbelow, referred to as an ECU) which controls the fuel injection quantity to the injector 8 so as to provide a predetermined air fuel ratio (A/F) with respect to air sucked into each of the cylinders. The ECU 9 determines the fuel injection quantity on the basis of the output signals of the AFS 2, a crank angle sensor 10, a start switch 11 and an engine-cooling water temperature sensor 12, and controls the pulse width of the fuel injection pulse signal to be supplied to the injector 8 in synchronism with the signal of the crank angle sensor 10. The crank angle sensor 10 may be of a well-known type of generating a rectangular waveform signal wherein it raises at the upper dead points (TDC) and falls at the lower dead points (BDC) with the revolution of the engine.
FIG. 2 is a block diagram for explaining in more detail the operation of the ECU 9.
At a revolution speed detecting section 9a, a revolution speed is obtained by measuring the period between adjacent TDCs of the rectangular waveform signal from the crank angle sensor 10. An averaged intake air quantity detecting section 9b operates to obtain the average value of output signals from the AFS 2 by the adjacent TDCs of the rectangular waveform output signal of the crank angle sensor 10. A basic pulse width subcalculation section 9c calculates a basic pulse width by dividing the average value of intake air quantity output signal of the average intake air quantity detecting section 9b by the output indicating the number of revolutions of the revolution speed detecting section 9a.
A warming-up correcting section 9d determines a correction coefficient in response to the temperature of cooling water to cool the engine, which is represented by the output of the cooling water temperature sensor 12. The basic pulse width obtained at the basic pulse width sub-calculation section 9c and the correction coefficient obtained at the warming-up correcting section 9d are added or multiplied at a correction value calculating section 9e to thereby obtain the pulse width for fuel injection.
On the other hand, a starting pulse sub-calculation section 9f calculates a starting pulse width on the basis of the detection signal of the water temperature sensor 12. A switch 9g selects either the injection pulse width or the starting pulse width upon receiving the output signal of the start switch 11 which detects the starting of the engine. A timer 9h is to effect a one-shot operation of the pulse width in time with a TDC falling point in the output signal of the crank angle sensor 10, whereby the injector 8 is actuated through an injector driving circuit 9i. The basic fuel injection quantity of the injector 8 corresponds to the intake air quantity per one revolution of the engine or the charging efficiency.
Generally, there takes place a pulsation of air or a reverse-flow of air in a low-speed-high-load area (1,000 -3,000 rpm and -50 mmHg-0 mmHg, in a case that no turbo charger is used) during the operation of the engine. In this case, there occurs an erroneous measurement by the AFS 2 due to the pulsation of air or the reverse flow of air.
FIG. 3 is a graph showing the relation of an air flow rate (the ordinate), boost pressure, i.e. a negative intake air pressure P (the abscissa) and a revolution speed (rpm) as parameters wherein the output of the AFS 2 (hot wire type) is sampled every 1 ms and the sampled output is converted into the flow rate wherein the value of the flow rate is averaged with respect to one air intake stroke.
As is clear from FIG. 3, the air flow rate A(n), when there occurs a reverse flow of air, shows a fairly large value in comparison with an actual air flow rate in the above-mentioned low-speed-high-load area in the engine operation. In order to eliminate such disadvantage, there has been considered that an upper limit value is determined on the extension line (indicated by a broken line) for each of the revolution speed levels at a point of a boost pressure of P=0 mmHg or a certain charging efficiency (i.e., 0.9) so that the value of intake air flow rate is clipped. Thus, by limiting the intake air flow rate A(n) to be a value which is subjected to the clipping treatment, an appropriate intake air flow rate can be obtained (when the engine is in a steady state) even in the above-mentioned low-speed-high-load area of the engine operation.
In the conventional control apparatus, however, there was found an overshoot in the air flow rate detected by the AFS 2 (as indicated by a solid line A in FIG. 4b) owing to an amount of air remaining in the surge tank and the intake manifold 5 when the automobile is rapidly accelerated, i.e. when the throttle valve is rapidly opened from the entirely closed state as shown by the solid line E in FIG. 4d. The detected air flow rate is not the value which is excessively detected due to the reverse flow of air, but is the actual flow rate. Accordingly, it is not suitable for clipping the air flow rate at the maximum air flow rate C (as indicated by one-dotted chain line) where the throttle valve is entirely opened. Namely, the conventional control apparatus wherein the upper limit is provided for each revolution speed level and the intake air quantity to the engine is clipped by the upper limit value, can not provide a good result when the engine is accelerated.